1. Field of the Invention
The present disclosure generally relates to semiconductor devices in SOI applications, and, more particularly, to tunable capacitors for FDSOI applications.
2. Description of the Related Art
At present, varactor diodes are used as tunable capacitors in VCO (voltage controlled oscillator) designs. VCOs have many applications, such as frequency modulation for FM transmitters and phase-locked loops. For example, phase-locked loops are used for frequency synthesizers that tune many radios, television sets and cellular telephones.
Conventionally, varactor diodes or varicap diodes are operated in a reverse biased state where no current flows. However, since the thickness of the depletion zone of the varactor diode varies with an applied bias voltage, the capacitance of the diode can be varied. The typical varactor may be formed by a depletion layer provided via a PN junction. As the depletion layer may be made of a MOS or a Schottky diode, the varactor may be implemented in CMOS or MMIC technologies.
U.S. Patent Publication No. 2009/0057746 shows the parallel integration of a varactor and a CMOS structure in SOI techniques.
The task of complying with Moore's law drives a scaling of semiconductor devices at present to more advanced technology nodes, particularly at 20 nm and beyond, in a continuous effort to develop more and more compact semiconductor devices. However, as induced by the scaling, shorts are caused between adjacent P-type back gates when a back bias for tuning the varactor is applied with shrinking separation between neighboring semiconductor devices because a separation between neighboring back gates used for tuning the varactor and possibly other CMOS devices in the vicinity to the varactor, such as conventionally used P-type varactors having a P-type back gate formed in a P-type base substrate of an SOI substrate, is hereby steadily reduced.
From the above, it is, therefore, desirable to provide a semiconductor device structure that avoids the problems as indicated above.